The present invention relates to a method of producing polycrystalline silicon film resistors used as components in semiconductor devices.
A polycrystalline silicon, or polysilicon, film is commonly used as a resistor in semiconductor devices. To meet current demands for higher operating speed and greater degree of integration of semiconductor devices, furthermore, a polycide film having a double-layer structure consisting of a polysilicon film and a silicide film are now being used as a gate and a resistor, instead of a polysilicon film. The polysilicon film used as wiring has a resistivity whose smallest value is 1.times.10.sup.-3 .OMEGA.cm and the polycide film has a smaller resistivity of 1.times.10.sup.-4 .OMEGA.cm.
When used as a resistor, however, such film must have a resistivity greater than the above values. For instance, a resistor used for SRAMs must have a resistance of 1 megohm to 1 gigaohm. A resistivity greater than 1.times.10.sup.-3 .OMEGA.cm is in many cases required even for other devices. When a high resistance is required, in particular, the polycide film that is used as the resistor is given a decreased width (W) and an increased length (L). For instance, when a resistor of 10 k.OMEGA. is to be obtained using a polycide film with a resistivity of 1.times.10.sup.-4 .OMEGA.cm, the resistor is given a length L=4000 .mu.(4 mm) if W=4 .mu. and its thickness t=0.1 .mu., which represents a very long section of wiring occupying a large area, resulting in an increase in the chip area.
In the case of polysilicon films, the resistivity changes depending upon the amount of doping; this makes it possible to form a high-resistance portion and a low-resistance portion in a single wiring. When a polysilicon resistor is used, therefore, only one polysilicon film suffices, and those film portions other than the resistor can be used as wiring portions and gate electrodes.
Such a structure is shown in FIG. 3 and includes a semiconductor substrate 21, an insulating film 22 on substrate 21 and a polysilicon film 23 formed on insulating film 22. High-resistance portions and low-resistance portions are formed in the same polysilicon film 23 without creating any problem in the connections between high-resistance portions and low-resistance portions.
When a polycide film is used, however, it is not permissible to arbitrarily control the resistivity of the polycide film; i.e., it is not permissible to form the low-resistance portions and the high-resistance portions simultaneously in one wiring. Therefore, a method has been employed to form the wiring in two layers as shown in FIG. 4 which illustrates an exemplary portion of an IC having a field-effect transistor formed in a semiconductor substrate 31 at a location where the surface of substrate 31 is exposed by an opening in an insulating film 32. The transistor is composed of a gate insulating film 33 and source and drain regions 35 and 36. Low resistance wiring portions, including the transistor gate electrode, are provided by polycide wiring 34 and wiring that must provide high resistance portions is composed of a polycrystalline silicon film 36. Reference numeral 37 denotes an aluminum film.
Thus, semiconductor devices having resistors employ a double-layer wiring consisting of a polycide wiring and a polycrystalline silicon film resistor, and their fabrication requires a large number of steps and a large number of lithography operations, with attendant high manufacturing cost. When polycide films are used, the double layer may have to be used though only one layer would have been needed if polysilicon film alone were used. Therefore, the number of steps increases to a considerable degree and, at the same time, mask alignment is needed, making it difficult to form fine patterns.
Moreover, the polysilicon films and the polycide films must be formed so as to reliably contact each other, and attention must be given to forming the contacts. Moreover, double-layer wiring makes the semiconductor device more rugged, and a special planarization is necessary to prevent breakage or short-circuits in the wiring.